1. Field of the Invention
The present invention relates to an InGaAsP type semiconductor laser device used for optical communication.
2. Description of the Related Art
In the past, as InGaAsP type semiconductor laser devices for optical communication, etc., a Buried Crescent (BC)-laser diode or a Buried Hetero-structure (BH)-laser diode have been known.
A BC-laser diode is shown in FIG. 4. In this device, a V-shaped groove 47 is formed in an InP substrate 41. A current blocking layer 42 including an n-type InP layer 42a and a p-type InP layer 42b is formed on a portion of the InP substrate 41, excluding the V-shaped groove 47. The current blocking layer 42 has a pn junction. A lower cladding layer 43, an active layer 44, an upper cladding layer 45, and a cap layer 46 are successively formed on the current blocking layer 42 with the V-shaped groove 47.
A BH-laser diode is shown in FIG. 5. In this device, a lower cladding layer 53, an active layer 54, an upper cladding layer 55, and a cap layer 56 are successively formed on an InP substrate 51. The active layer 54, the upper cladding layer 55, and the cap layer 56 are formed in a stripe shape on the lower cladding layer 53 to provide a ridge portion. The ridge portion is buried with the current blocking layer 52 including an n-type InP layer 52a and a p-type InP layer 52b. The current blocking layer 52 has a pn junction.
There is the following problem in the above-mentioned structure in which an electric current is blocked by using a pn junction: A parasitic capacitance is increased due to a depletion layer formed around the pn junction, resulting in the increase in the capacitance of the device, making it difficult to realize a high-speed response to a high frequency electronic signal.
In order to realize a high-speed response, a semiconductor laser device with the following structure has been suggested.
A BH-laser diode in which a high-speed response is realized is shown in FIG. 6. In this device, grooves 67 are formed in a current blocking layer 62 including an n-type InP layer 62a and a p-type InP layer 62b so as to reach a substrate 61. SiO.sub.2 films 68 are formed on the upper face of the current blocking layer 62 and on the inner surface of the grooves 67. In this figure, the reference numerals 63, 64, 65, and 66 denote a lower cladding layer, an active layer, an upper cladding layer, and a cap layer, respectively. In this structure, an electric current is blocked by the SiO.sub.2 film 68 so as to be confined in the ridge portion. Thus, the capacitance of the device is reduced, compared with the case where the pn junction is used for current blocking, making possible a high-speed response. This high-speed response using the groove structure can also be applied to a device with a BC-laser diode.
In addition, a BH-laser diode as shown in FIG. 7 has been suggested. In this device, a p-type InP layer 70, an SiO.sub.2 film 71, and a heat-resistant resin film with high electrical resistivity 72 are successively formed so as to bury a ridge portion including an active layer 74. In this structure, an electric current is blocked by using the SiO.sub.2 film 71 and the heat-resistant resin film with high electrical resistivity 72 to be confined in the active layer 74, instead of using a pn junction. Moreover, the SiO.sub.2 film 71 and the heat-resistant resin film with high electrical resistivity 72 are provided with a thickness of 2 .mu.m or more. In this figure, the reference numerals 73, 75, 76, and 77 denote a lower cladding layer, an upper cladding layer, a cap layer, and a substrate, respectively.
Moreover, an AlGaAs type semiconductor laser device as shown in FIG. 8 has been suggested. In this device, a lower cladding layer 83, an active layer 84, an upper cladding layer 85, and a cap layer 86 are successively formed on a GaAs substrate 81. The upper portion of the upper cladding layer 85 and the cap layer 86 are formed in a stripe shape to provide a ridge portion. This ridge portion is buried with a heat resistant resin film with high electrical resistivity 87 so that the ridge portion and the heat-resistant resin film with high electrical resistivity 87 form a flat surface.
There are the following problems in the semiconductor laser devices with the above-mentioned structures.
In the semiconductor laser devices as shown in FIGS. 6 and 7, the active layer is formed in a stripe shape by etching. Because of this, the side faces of the active layer are exposed to air, resulting in the deterioration of the crystallinity of the active layer in the vicinity of the side faces of the ridge portion. Even after the ridge portion is buried with the current blocking layer 62, a plurality of non-radiative recombination centers are formed due to this deterioration of crystallinity. Thus, the reliability of the device is decreased.
In the semiconductor laser device as shown in FIG. 7, since the SiO.sub.2 film 71 and the heat-resistant resin film with high electrical resistivity 72 are provided with a large thickness, the InP substrate 77, the SiO.sub.2 film 71, and the heat-resistant resin film with high electrical resistivity 72 will have different thermal expansion coefficients. Because of this, large amounts of stress and strain are generated between the InP substrate 77, and the SiO.sub.2 film 71 and the heat-resistant resin film with high electrical resistivity 72, rapidly deteriorating the device.
In the AlGaAs type semiconductor laser device as shown in FIG. 8, the upper face of the ridge portion and the heat-resistant resin film with high electrical resistivity 87 should form a flat surface so as to be bonded to a submount. Because of this, the thickness of the heat-resistant resin film with high electrical resistivity 87 is about 2 .mu.m. In the case where this structure is applied to an InGaAsP type semiconductor laser device, great amounts of stress and strain are generated between the substrate 81 and the heat-resistant resin film with high electrical resistivity 87 due to the difference of thermal expansion coefficient thereof, rapidly deteriorating the device.